It is customary, for example, in the technology of printing and packaging, to control processing operations on a travelling material web by providing printing control marks on the web which can be sensed by photocells or other optical devices.
These markings or control marks are most frequently printed in a colour tone such as black which contrasts with the surroundings, and they are applied on such zones of the material web where no other printed markings or patterns are present. The markings can also be made with magnetically sensible material or with the help of marks sensible by mechanical means, e.g. holes, crease lines, slots, etc.
These control marks are used e.g. in multicolour printing to adapt the position of the patterns printed in different colours to one another so that the different colour patterns come to lie accurately on top of one another. A second, similar usage, where it is intended to add print to a previously preprinted material web at a certain point in the printing pattern, e.g. a date marking or the like, or where it is intended to arrange a printed pattern and a crease line pattern facilitating the folding in register with each other. Further fields of application exist, for example, where a packing material web is to be advanced in a packing machine or the like over exactly the length of a pattern so as to obtain, on the one hand, the printed pattern in the same position on all packages, and to ensure, on the other hand, that the crease line pattern coincides with the forming device of the packaging machine so that the folding of the material takes place along the crease lines which are predetermined in their position.
As mentioned previously, it has been necessary up to now either to provide the web with a longitudinal blank band which may only contain control markings or else it has been necessary, in each case, to place the control markings in a relatively large blank zone, the photocell device being activated only when it is certain that the area with the zone or "window" has been reached.
Such blank bands or zones naturally disturb the general picture of the design and it is desirable, therefore, to accommodate the control markings within the existing design or on very small blank surfaces which, in principle, only comprise the control markings. However, this has given rise to great problems since the elements adapted to sense the markings respond to parts of the design as well as control marks, with the consequence that undesired adjustments or processing operations are carried out following the detection of "false signals."